The present invention relates to an apparatus for measuring the concentration of radioactivity of a radioactive fluid flowing in a pipe and, more particularly, to an apparatus for measuring the concentration of radioactivity in the water discharged from a nuclear power plant.
In nuclear power plants and other facilities treating radioactivity, it is necessary to measure the concentration of the radioactivity of liquids flowing in various pipes. For instance, in the nuclear power plants, any abnormality in the plant is detected through the measurement of the radioactivity, to prevent dangerous wasting of liquid of high radioactivity concentration to the outside of the plant.
Japanese Patent Laid Open Nos. 55581/77 and 137595/77 disclose apparatus for measuring the radioactivity concentration in the radioactive fluids flowing in the pipes of nuclear power plants.
In these apparatus, the water discharge pipe through which the radioactive fluid as the measuring object flows is arranged to extend vertically. A measurement pipe, which has a diameter greater than the water discharge pipe, is arranged coaxially with the water discharge pipe so that the radioactive liquid may naturally fall into the measurement pipe along the axis of the latter. The end of the water discharge pipe extends into the measurement pipe, so that the radioactive liquid does not make direct contact with the inner surface of the measurement pipe. In consequence, the accumulation of radioactive substances on the pipe surface is avoided. A radioactive ray detector is disposed at the outside of the measurement pipe, and measures the radioactivity concentration in the radioactive liquid without making contact with the measurement pipe. The water which has passed through the measurement pipe is discharged through a discharge opening via a water outlet pipe provided at the outlet side of the measurement pipe and then a discharge water stop valve.
In this arrangement, however, it is often experienced that the water level rises within the length of the measurement pipe when a large flow resistance is imposed by the water pipe due to, for example, a too large distance between the water discharge opening and the position where the detector is situated. In such a case, it is difficult to maintain a steady flow of water without permitting the direct contact between the water and the inner surface of the measurement pipe.
In this known measuring apparatus, therefore, it is necessary to maintain the air in the measurement pipe at the atmospheric pressure. It is, therefore, necessary to install the detector at the outlet portion of the water outlet pipe disposed at the outlet side of the measurement pipe or to dispose a tank having a free water surface beneath the region where the water falls without contacting the inner surface of the measurement pipe. It is, however, difficult to obtain sufficient installation spaces for the detector and the water tank at such locations. This difficulty or problem equally applies to the measurement of radioactivity concentration in ordinary pipes other than the water discharge pipe.